How to Calculate Thermocouple Temperature Based on Output Millivoltage?

Instrument Integral December 28, 2023

How to Calculate Thermocouple Temperature Based on Output Millivoltage?

The National Institute of Standards and Technology (NIST) serves as the United States' standards-setting agency and has established the output millivoltage values for all types of thermocouples across their temperature range. These values are compiled in "Thermocouple Reference Tables," providing the corresponding thermocouple output millivoltage for every degree of temperature.

Thermocouple Reference Tables Link Here

A thermocouple consists of two junctions – the measuring junction and the reference junction – connected in opposition. The resulting millivoltage (VD) is obtained by measuring the difference between the millivoltages generated by these two opposing junctions. When a meter is connected across the thermocouple, VD is the recorded millivoltage.

Determining the Measuring Junction Temperature

1. Measure VD Millivoltage:

Measure the millivoltage (VD) as described above.

2. Measure Reference Junction Temperature:

Use a thermometer to measure the actual temperature of the reference junction.

3. Consult Thermocouple Table:

Refer to the table for the specific thermocouple being used and find the millivoltage produced at the reference junction temperature.

4. Calculate Total Millivoltage:

Add the millivoltage obtained from the table to the measured VD to obtain a total.

5. Find Corresponding Temperature:

Locate the total millivoltage in the reference table. The corresponding temperature is the temperature of the measuring junction.

Example Calculations

Example #1: Type “T” Thermocouple

Measured VD = 3.41 mV
Reference Junction Temperature = 22°C (71.6°F)
From the table, 22°C = 0.87 mV.
Adding 0.87 mV to 3.41 mV = 4.28 mV.
In the table, the corresponding temperature is 100°C (212°F), which is the temperature of the measuring junction.

Example #2: Type “T” Thermocouple

Measured VD = 4.47 mV
Reference Junction Temperature = -5°C (23°F)
From the table, -5°C = -0.193 mV
Adding -0.193 mV to +4.47 mV = +4.28 mV
In the table, the corresponding temperature is 100°C (212°F), indicating the temperature of the measuring junction.

Metric/English Scale Conversion

°C = (°F – 32) / 1.8
°F = (1.8 * °C) + 32

Note: Positive Leg (P), Negative Leg (N)

Additional Information:

Standard-grade wire is generally accurate for most applications. Premium-grade wire has stricter control over purity and composition, resulting in a millivoltage output closer to the NIST standard chart for enhanced accuracy.

Note: Individual thermocouple units may be calibrated by measuring their output at various known temperatures, creating an error correction chart for NIST traceability.

Temperature